A wheel rolling bearing unit for supporting a wheel is used for a vehicle, such as an automobile. This wheel rolling bearing unit is in some cases equipped with an inner shaft 101 having a flange 100 on which a brake rotor and a wheel of a vehicle are mounted; an outer ring 102 secured to the side of the vehicle and disposed concentrically with the inner shaft 101; a plurality of balls 103 rotatably arranged in double rows between the inner shaft 101 and the outer ring 102; sealing members 104 for hermetically sealing the annular clearance formed between the inner shaft 101 and the outer ring 102; and cages 105 for retaining the plurality of balls 103 at predetermined intervals in the circumferential direction as shown in FIG. 5 (for example, refer to Patent Document 1).
Grease for lubrication is sealed between the inner and outer rings of the above-mentioned wheel rolling bearing unit. The sealing member 104 has an annular seal lip which is secured to the side of the outer ring and the tip end of which is pressed to the outer peripheral surface of the inner shaft 101 and is made slide contact therewith, and this seal lip hermetically seals the annular clearance formed between the inner and outer rings, thereby preventing the grease from leaking to the outside.
In the above-mentioned wheel rolling bearing unit, if grease leak occurs from the sealing member 104 on the side of the flange, there is a danger that the leaked grease may flow along the flange 100 and be attached to the brake rotor. Hence, to prevent grease leak, measures have been taken, for example, the interference of the sealing member 104 (the level of the pressing force at the seal lip) is set so as to be relatively large to further enhance the hermetic sealing performance of the sealing member 104.